For the purpose of surveillance or reconnaissance, it is desirable for an observer to be able to see a subject without the subject being able to see the observer. The observer and subject need not be people: the observer might be a camera or other light detecting device; the subject might be anyone or anything that the observer is interested in observing.
Devices have been designed expressly for restricting light incident on them to transmission in one direction only. With an ideal device, it would be impossible for the subject to see the observer at all, while the observer would still see the subject clearly. The current state of the art presents two types of optical devices for approaching this ideal: the two-way mirror and the neutral density filter.
A two-way mirror consists of a transparent panel with reflective material that partially transmits and partially reflects light incident from the subject side, while transmitting most light incident from the observer side. Since it allows light to be transmitted from the observer to the subject, the two-way mirror is installed so that the observer is in a darker area than the subject. Under these conditions the light reflected back onto the subject overwhelms the light transmitted from the observer, allowing one-way observation.
A disadvantage to the use of two-way mirrors results from their common use in public. Two-way mirrors are so commonly used for optical surveillance that members of the general public are often suspicious of any mirrored surface in a public place. Remaining inconspicuous to a potential subject is important in the field of covert surveillance. In many cases this is impossible with a two-way mirror.
Neutral density filters are an alternative to two-way mirrors in the current state of the art. A neutral density filter consists of a combination of metals such as aluminum, nickel or chromium that have been evaporated onto a glass substrate. With different combinations of these metals, the neutral density filter can be made reflective, or absorptive. In order to avoid the previously mentioned disadvantage of reflective surfaces, the neutral density filters used in surveillance are usually made absorptive.
An absorptive neutral density filter works by absorbing a broad spectrum of incident light. As a result, the filter generally looks like a dull, opaque surface. The absorption spectrum for light transmitted through a neutral density filter, however, does not vary much with the direction in which light is transmitted. The subject appears dim or dark to the observer. Neutral density filters are used when it is very important for the observer to be inconspicuous, in which case the comparatively high optical quality of a two-way mirror is exchanged for the less noticeable alternative of a neutral density filter.
Other approaches have been proposed for covert surveillance windows. U.S. Pat. No. 6,185,039 to Allen et al. discloses an “Infrared Selective Reflective Polarizing Element” (ISRPE). The ISRPE includes two linear polarizing components, which may be rotated with respect to a common in-plane axis in order to control the amount of light transmitted. The ISRPE also includes an infrared reflective or absorptive component, for reflecting or absorbing infrared wavelength light incident to the ISRPE. Crossed-polarizers, however, must be mechanically rotated in order to control the amount of light transmitted, and do not allow covertness when in an open configuration.
U.S. Pat. No. 6,064,523 to Budd et. al discloses an “Apparatus For Polarization Conversion”, whereby S polarized light may be converted to P polarized light, or vice versa. Budd et al. does not, however, disclose how such an apparatus could be used to selectively block light, for example, for use in covert surveillance.